Related fields include semiconductor fabrication; specifically, etching of structures having multiple layers of different compositions, and most particularly where some of the layers include titanium or a titanium compound.
In fabricating semiconductor devices, the formation of various material layers on a substrate is followed by patterning the layers to form structures (e.g., transistors, capacitors, interconnects, and others). “Etching,” used herein to mean “removal of at least part of a layer or structure,” is often part of the patterning process. Etching can include dry processes (e.g., plasma etching, plasma-less gas etching, sputter etching, ion milling, reactive ion etching (RIE)) or wet processes (e.g., applying an acid, base, or solvent to dissolve part of the structure, or an abrasive formulation to polish away part of the structure).
Where only one material is exposed to the etchant, some primary concerns may be etch rate, feature resolution, and residues left behind by the process. Where more than one material is exposed to the etchant, these concerns are also important, but the constraints may multiply. For example, the intent may be to etch one or more of the materials to a greater depth while etching the other materials to a lesser depth (“selective etching”). In highly selective etching, the intent may be to leave one or more other materials substantially intact despite exposure to the etchant. In other situations, it may be advantageous to etch multiple materials to within 5-15% of the same depth (“uniform etching”).
Besides the composition of the layer, the formation method of the layer may influence its etching behavior. For example, some materials deposited by atomic layer deposition (ALD) are more difficult to etch than the same material deposited by physical vapor deposition (PVD).
As an example, consider a metal stack that includes tungsten (W) and a titanium-based material such as titanium nitride (TiN), titanium carbide (TiC) or titanium aluminide (TiAl). In some applications, it is desirable to etch the W and the Ti-based material(s) non-selectively, i.e., at about the same rate, to recess them all to the same depth by the same process. Solutions of hydrogen peroxide (H2O2), such as commercial sulfuric acid/peroxide mixture (SPM), Standard Clean 1 (SC-1), or Standard Clean 2 (SC-2), etch all these materials. However, they etch W much faster than they etch the Ti-based materials, producing a selective etch rather than the desired uniform etch.
Plasma etching can etch W and many Ti-based materials at similar rates, but TiC is an exception. Plasma etching of TiC creates excess residues that block further etching of the materials underneath the residue.
Therefore, a need exists for formulations and methods to simply and cost-effectively etch W and Ti-based materials to a common recess depth.